This PR implements a class for DB models to typify and unify model objects:

• encapsulate all used DB data types into static properties
• encapsulate all used DB service field names into static properties
• add immutability of DB model instances for security reasons
• add access methods for model fields, triggers, constraints, and indexes as an easier way to get appropriate fields
• add method to clone the model as a way to have a mutable object without a need to change the main one
• add a simple validation of model field types on the stage of making an instance

The PR is part of the refactoring and doesn't have the changes of the main logic and functionality